The adaptive immune system protects the host from a second infection by generating infection-specific lymphocytes called memory cells. Memory cells come from a much larger population of effector cells, which are generated from rare nave cells that proliferate after antigen binds to their antigen receptors. The objective of this projec is to understand CD4+ memory T cell formation. This is a significant issue because CD4+ T cells control certain intracellular infections by using T cell antigen receptors (TCR) to recognize microbial peptides bound to MHCII molecules on infected host cells. We made an important step in the right direction during the last funding period by developing a peptide:MHCII tetramer-based cell enrichment method to track CD4+ memory T cell formation in vivo. We found that infection with Listeria monocytogenes (Lm) caused a population of ~100 Lm peptide:MHCII-specific nave CD4+ T cells to proliferate and form three types of effector cells: Th1 cells and two types of follicular helper cells, Tfh and GC-Tfh cells. Using a novel limiting dilution adoptiv transfer method, we found that different single nave cells produced effector cell populations with different Th1/Tfh/GC-Tfh patterns and that clonal effector cell populations yielded smaller memory cell populations with a similar pattern. Importantly, the effector and memory cell subset pattern produced by a nave clone was in part an intrinsic feature of its T cell receptor (TCR). I is now critical to understand the mechanisms that cause individual nave cells to bifurcate down the Th1 or Tfh effector cell pathways since this decision sets the memory cell pattern. Using robust single cell methods, we will address the hypothesis that TCR signal strength, dendritic cell subsets and innate cytokines influence how nave CD4+ T cells proliferate, undergo apoptosis, and form Th1, Tfh, or GC-Tfh effector cells. Since all clonal effector cell populations produce memory cell populations of a smaller size, it is critical to understand why most of the effector cells die and some survive as memory cells. We will perform experiments to determine whether all effector cells have a chance of becoming memory cells or whether a small subset of effector cells is fated to become memory cells as a result of an asymmetric cell division or by switching on a catabolic pathway. This research is innovative because it will employ novel technology to track the fates of single CD4+ T cells at key checkpoints.